YOUR MIND AND HOW TO USE IT/PART 3
CHAPTER III.
The Great Nerve Centers.
THE
great nerve centers which play an important part in the production and
expression of mental states are those of the brain and spinal cord,
respectively.
The
Spinal Cord.
The
spinal cord is that cord or rope of nerve substance which is inclosed in the
spinal column or "backbone." It leaves the lower part of the skull
and extends downward in the interior of the spinal column for about eighteen
inches. It is continuous with the brain, however, and it is difficult to
determine where one begins and the other ends. It is composed of a mass of gray
matter surrounded by a covering of white matter. From the spinal cord, along
its length, emerge thirty-one pairs of spinal nerves which branch out to each
side of the body and connect with the various smaller nerves, extending to all
parts of the system. The spinal cord is the great central cable of the nervous
telegraphic system, and any injury to or obstruction of it cripples or paralyzes
those portions of the body the nerves of which enter the spinal cord below
the seat of the injury or obstruction. Injuries or obstructions of this kind
not only inhibit the sensory reports from the affected area, but also inhibit
the motor impulses from the brain which are intended to move the limbs or parts
of the body.
The
Ganglia or "Tiny Brains."
What
are known as ganglia, or tiny bunches of nerve cells, are found in various
parts of the nervous system, including the spinal nerves. These groups of nerve
cells are sometimes called "little brains," and perform quite
important offices in the mechanism of thought and action. The spinal ganglia
receive sensory reports, and issue motor impulses, in many cases, without
troubling the central brain regarding the matter. These activities are known as
"reflex nervous action."
Reflex
Action.
What
is known as reflex nervous action is one of the most wonderful of the
activities of the nervous and mental mechanism, and the knowledge thereof
usually comes as a surprise to the average person, for he is generally under
the impression that these activities are possible only to the central brain. It
is a fact that not only is the central brain really a trinity of
three brains, but that, in addition to these, every one has a great number
of "little brains" distributed over his nervous system, any and all
of which are capable of receiving sensory reports and also of sending forth
motor impulses. It is quite worth while for one to become acquainted with this
wonderful form of neuro-mental activity.
A
cinder enters the eye, the report reaches a ganglion, a motor impulse is sent
forth, and the eyelid closes. The same result ensues if an object approaches
the eye but without actually entering it. In either case the person is not conscious
of the sensation and motor impulse until the latter has been accomplished. This
is reflex action. The instinctive movement of the tickled foot is another
instance. The jerking away of the hand burnt by the lighted end of the cigar,
or pricked by the point of the pin, is another instance. The involuntary
activities, and those known as unconscious activities, result from reflex
action.
More
than this, it is a fact that many activities originally voluntary become what
is known as "acquired reflexes," or "motor habits," by
means of certain nervous centers acquiring the habit of sending forth certain
motor impulses in response to certain sensory reports. The familiar movements
of our lives are largely performed in this way, as, for instance, walking, using
knife and fork, operating typewriters, machines of all kinds, writing, etc. The
squirming of a decapitated snake, the muscular movements of a decapitated frog,
and the violent struggles, fluttering, and leaps of the decapitated fowl, are
instances of reflex action. Medical reports indicate that in cases of
decapitation even man may manifest similar reflex action in some cases. Thus we
may see that we may feel and will by means of
our "little brains" as well as by the central brain or brains.
Whatever mind may be, it is certain that in these processes it employs other
portions of the nervous system than the central brain.
The
Three Brains.
What
is known as the brain of man is really a trinity of three brains, known
respectively as (1) the medulla oblongata, (2) the cerebellum,
and (3) the cerebrum. If one wishes to limit the mental activity to
conscious intellectual effort, then and then only is he correct in considering
the cerebrum or large brain as "the brain."
The
Medulla Oblongata.—The medulla oblongata is an enlargement of the spinal cord at the
base of the brain. Its office is that of controlling the involuntary activities
of the body, such as respiration, circulation, assimilation, etc. In a broad
sense, its activities may be said to be of the nature of highly developed and
complex reflex activities. It manifests chiefly through the sympathetic nervous
system which controls the vital functions. It does not need to call on the
large brain in these matters, ordinarily, and is able to perform its tasks without
the plane of ordinary consciousness.
The
Cerebellum.—The cerebellum, also known as "the little brain," lies
just above the medulla oblongata, and just below the rear portion of the
cerebrum or great brain. It combines the nature of a purely reflex center on
the one hand, with that of "habit mind" on the other. In short, it
fills a place between the activities of the cerebrum and the medulla oblongata,
having some of the characteristics of each. It is the organ of a number of
important acquired reflexes, such as walking, and many other familiar muscular
movements, which have first been consciously acquired and then become habitual.
The skilled skater, bicyclist, typist, or machinist depends upon the cerebellum
for the ease and certainty with which he performs his movements "without
thinking of them." One may be said never to have thoroughly acquired a set
of muscular movements such as we have mentioned, until the cerebellum has taken
over the task and relieved the cerebrum of the conscious effort. One's
technique is never perfected until the cerebellum assumes control and direction
of the necessary movements and the impulses are sent forth from below the plane
of ordinary consciousness.
The
Cerebrum.—The cerebrum, or "great brain" (which is regarded as "the
brain" by the average person), is situated in the upper portion of the
skull, and occupies by far the larger portion of the cavity of the skull. It is
divided into two great divisions or hemispheres. The best of the modern
authorities are agreed that the cerebrum has zones or areas of specialized
functioning, some of which receive the sensory reports of the nerves and organs
of sense, while others send forth the motor impulses which result in voluntary
physical action. Many of these areas or zones have been located by science,
while others remain as yet unlocated. The probability is that in time science
will succeed in correctly locating the area or zone of each and every class of
sensation and motor impulse.
The
Cortex.
The
area of thought, memory, and imagination has not been clearly located, except
that these mental states are believed to have their seat in the cortex or
outer thin rind of gray brain matter which envelopes and covers the mass of
brain substance. It is, moreover, considered probable that the higher processes
of reasoning are performed in or by the cortex of the frontal lobes. The cortex of a person of average intelligence, if spread out on a
flat surface, measures about four square feet. The higher the degree of
intelligence possessed by a lower animal or human being, as a rule, the deeper
and more numerous are the folds or convolutions of the cortex, and the finer
its structure. It may be stated as a general rule, with but very few
exceptions, that the higher the degree of intelligence in a lower animal or
human being, the greater is the area of its cortex in proportion to the size of
the brain. The cortex, it must be remembered, is folded into deep furrows or
convolutions, the brain in shape, divisions, and convolutions resembling the inner
portion of an English walnut. The interior of the two hemispheres of the
cerebrum is composed largely of connective nerves which doubtless serve to
produce and maintain the unity of function of the mental processes.
While
physiological psychology has performed great work in discovering brain-centers
and explaining much of the mechanism of mental processes, it has but touched
the most elementary and simple of the mental processes. The higher processes
have so far defied analysis or explanation in the terms of physiology.
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